bzoj5305 [Haoi2018] 苹果树

传送门：https://www.lydsy.com/JudgeOnline/problem.php?id=5305

【题解】

# include <bits/stdc++.h>
using namespace std;

const int M = 2010;

int n, mod;
int C[M][M], fac[M];

int main() {
    cin >> n >> mod;
    fac[0] = 1;
    for (int i=1; i<=n; ++i) fac[i] = 1ll * fac[i-1] * i % mod; 
    C[0][0] = 1;
    for (int i=1; i<=n; ++i) {
        C[i][0] = 1;
        for (int j=1; j<=i; ++j) {
            C[i][j] = C[i-1][j] + C[i-1][j-1];
            if(C[i][j] >= mod) C[i][j] -= mod;
        }
    }
    int ans = 0;
    for (int i=2; i<=n; ++i)
        for (int sz=1; sz<=n-i+1; ++sz) {
            int tot = 1ll * fac[n-sz-1] * i % mod * (i-1) % mod * fac[sz] % mod * C[n-i][sz-1] % mod;
            ans += 1ll * tot * sz % mod * (n-sz) % mod;
            if(ans >= mod) ans -= mod;
        }
        
    cout << ans % mod << endl;    
    return 0;
}